Magneto-optical data storage combines the advantages of magnetic data storage (including the advantage that data can be deleted and newly written on the same carrier), with the advantages of the purely optical data storage (which include avoidance of unwanted data deletion after a crash of the data head onto the data carrier). In addition, with magnetic optical storage, very high storage densities can be achieved which are of the order of magnitude of 5.times.10.sup.8 bits/cm.sup.2 and which permit the manufacture of storage disks.
Magneto-optical storage relies upon the phenomenon that polarized light experiences a rotation of its plane of polarization in passing through a magnetic field (the Faraday Effect) as well as in reflecting from a ferromagnetic surface (the magneto-optical Kerr Effect). The magnitude of the rotation of the plane of polarization for a given material is a monotonic function of the magnetization. In order to achieve such polarization rotation, a magnetic layer may be sputtered on a substrate. The magnetic layer may consist primarily of binary, ternary or quaternary alloys of the elements of the group of rare earths (.sup.59 Pr-.sup.70 Yb) and the group of transition metals (.sup.26 Fe-.sup.28 Ni) (See Mark H. Kryder, J. Vac. Sci. Technol., A4 (3), 1986, page 558). These alloys are applied in a dc or rf sputtering process (See Hong, Gyorgy, van Dover, Nakahara, Bacon, Gallagher, "DC magnetron and diode-sputtered polycrystalline Fe and amorphous Tb(FeCo) films: Morphology and magnetic properties," J. Appl. Phys. 59 (2), 1986, pages 551-556) in the form of an amorphous thin film on suitable substrates. Initially, a Curie and compensation temperature of the ferrimagnetic layer in the vicinity of 150.degree. C. is aimed for. The magneto-optical Kerr effect,
should be as large as possible. This means that the angle through which the light polarization is rotated, which depends on the orientation of magnetization of the reflecting region, should be large. With a laser the layer is heated to the region of the ferrimagnetic system compensation point and an orientation impressed upon it in a magnetic field. For reading out this information, laser light of markedly lower intensity and linear polarization is employed. The orientation is rotated as a consequence of the magneto-optical Kerr effect. The direction of rotation is determined with an analyzer; it contains the stored information (See Hans-Gerd Severin, "Sputtern, Die Erzeugung duinner Schichten," Zeitschrift Physik in unserer Zeit, 17, 1986 pages 71-79, and in particular pages 77 and 78). A big problem fabricating this type magneto-optical storage device is the oxidation sensitivity of the magnetic layer. The magnetic layer must therefore be protected by additional layers against corrosion and abrasion. Metal layers or dielectric layers, for example Al, AlN, or Si.sub.3 N.sub.4, have been employed for this protective purpose.
While the starting materials of the alloys (i.e. the rare earths and the transition metals) have a ferromagnetic order, the alloys of these starting materials have a ferrimagnetic order (i.e. the rare earths and transition metals, which for their part are ferromagnetically ordered, couple mutually with each other antiferromagnetically).
The magnetization behavior of the alloy layers corresponds to that of soft magnets. Apart from a high squareness (.about.1) they have a strong magnetic anisotropy of (k.sub.u .about.10.sup.6 ergs/cm.sup.3) perpendicularly to the surface of the layer. Additional characteristic quantities for the magnetic behavior of thin layers of rare earth and transition metals are coercive field strengths H.sub.c, for example at room temperature, compensation temperature T.sub.comp, and Curie temperature T.sub.c. The coercive field strength is here defined as that negative field strength H.sub.c which brings the magnetic flux density B again to zero. The Curie temperature T.sub.c is the temperature at which the ferromagnetic properties of a material vanish. Above the Curie temperature T.sub.c, the spontaneous magnetization vanishes entirely and the previously ferromagnetic material behaves like a paramagnetic material. The compensation temperature T.sub.comp is that temperature at which the magnetization of an alloy is zero, specifically due to the addition of the positive magnetization of the one component of the alloy to the negative magnetization of another component of the alloy. At the compensation temperature T.sub.comp the magnetization likewise vanishes completely; however, the magnetic order of the individual materials is retained, in contrast to the behavior of these materials at the Curie temperature
Since rare earth and transition metals differ with respect to direction of magnetization and also with respect to the temperature dependence of the magnetization magnitude, the magnetization of each overall alloy has a characteristic temperature dependence. It is therefore of great importance to determine or define the fractions of the rare earth and transition metals within an alloy. Sputter systems exist in which targets composed of plates of the individual elements (so-called "mosaic targets") are used in order to determine the rare earth and transition metal fractions in the alloy layers. As studies of such targets have shown, however, very often composition fluctuations due to fabrication conditions are found. In addition, as the target erodes, the particle radiation characteristic of the racetracks varies with different distribution of the individual elements in the plasma. These effects lead to fluctuations or to longterm drift of the fabricated function layers.
It is also conventional to determine the particular fractions of the starting materials by using appropriate sinter and alloy targets (See H. P. D. Shieh, Dissertation, Carnegie-Mellon University, Pittsburgh, CA {SIC: PA}, USA 1987). As investigations have shown, the
temperature (and with it the room temperature-coercive field strengths) can be varied within certain limits by varying the sputter gas pressure and, in the case of diode sputtering, also by substrate biasing.
However, conventional sputtering methods have the disadvantage that changes in the processing pressure during magnetron sputtering lead to undesirable changes in the microstructure of the thin alloy layers. In addition, the specific rates in diode sputtering are comparatively low and very high target voltages are required. Furthermore, the application of a substrate bias in a production installation is technically difficult and, in the case of an rf bias, frequently leads to destabilization of the processes. In addition, since high concentrations of Ar atoms are built into the layers, the magnetic properties of the thus obtained alloy layers are not stable in the long term.
It is also conventional to change the relative position of magnet system and target plate continuously to make more uniform the consumption of target plates (See German Pat. Nos. DE-OS 27 35 525 and DE-OS 30 47 113 or U.S. Pat. No. 4,426,264). These conventional methods, however, do not allow precise determination of the composition of the alloys to be applied.
It is also known that the angular distribution of the emerging target atoms in multi-component targets can differ among the individual components, and that the installation geometry will influence the layer composition (Physik in unserer Zeit, loc.cit., pp. 74-75).
Further, a conventional sputtering cathode is suitable for magnetic as well as for non-magnetic target materials. This sputtering cathode permits coating of large areas by continuous processing of the substrates and leads to a very uniform layer thickness distribution, and yet can be operated with only a single current supply (See European Pat. Application 0 253 344 A2). This sputtering cathode has at least three magnet units, one within the other, and is equipped with a mechanical adjusting device by which the magnetic flux between a magnet yoke and at least one magnet unit can be varied relative to the magnetic flux between the magnet yoke and the remaining units.
Finally, a device for improving the electro-optical effect of a ferroelectric thin film is known which employs oxides of Pb, Ti, and La, wherewith the molar ratio of Pb to Ti is fixed within a specific range (See Patent Abstracts of Japan, June 8, 1984, Vol. 8, No. 122, JP-OS 59-35098). Herein powdered oxides of Pb, Ti, and La are mixed with each other, calcinated, and sputtered, with the powder serving as the target, in order to form a thin film on a substrate. The molar ratio of Pb to Ti is preferably 0.65&lt;Pb/Ti&lt;0.90. The electro-optical effect of the resulting ferroelectric thin monocrystalline film is thereby improved and such film can be used as light-switching material.